1. Field of the Invention
The invention disclosed in this specification and the like relates to a driving method of a semiconductor device.
2. Description of the Related Art
In recent years, flat panel displays such as liquid crystal displays have been widespread. In each of pixels provided in the row direction and the column direction in a display device such as a liquid crystal display, a transistor serving as a switching element, a liquid crystal element electrically connected to the transistor, and a storage capacitor connected to the liquid crystal element in parallel are provided.
As a semiconductor material for forming a semiconductor film of the transistor, a silicon semiconductor such as amorphous silicon or polysilicon (polycrystalline silicon) is generally used.
Metal oxides having semiconductor characteristics (hereinafter referred to as oxide semiconductors) can be used for semiconductor films in transistors. For example, techniques for forming transistors including zinc oxide or an In—Ga—Zn-based oxide semiconductor are disclosed (see Patent Documents 1 and 2).
In the storage capacitor of the display device, a dielectric film is provided between a pair of electrodes at least one of which is formed, in many cases, using a light-blocking conductive film partly serving as a gate electrode, a source electrode, a drain electrode, or the like of the transistor.
With an increase in the capacitance value of the storage capacitor, the alignment of liquid crystal molecules of the liquid crystal element can be kept constant for a longer period in the state where an electric field is applied, leading to a reduction in power consumption of the display device.
For example, one of methods for increasing the charge capacity of the storage capacitor is to increase the area occupied by the storage capacitor, specifically, to increase the area of a region where the two electrodes of the storage capacitor overlap with each other. However, when the area of the light-blocking conductive film is increased to increase the area of the region where the two electrodes overlap with each other, the aperture ratio of a pixel is lowered and thus display quality of an image is degraded.
Thus, a technique has been disclosed, which enables desired charge capacity to be ensured without a decrease in the aperture ratio in such a manner that a light-transmitting storage capacitor formed using a light-transmitting material is provided in a display device (see Patent Document 3).